Well I think what you want is a subreddit that answers political theory questions, but I suppose that politics is always half history anyways - I'm still mad at you that I have to go through my books to find my Marxism 101 book but I'm doing this anyways because I'm pedantic and ridiculous.
The general idea is that Marx wrote das capital (Vol.1 1867, Vol.2 1885, Vol.3 1894) in Western Europe during the industrial revolution, and this is sort of the basis of it, that Russia is much more agrarian, and still has a lot of basis in feudalism, whereas West Europe was developing more of a middle class and nouveau riche. You also have to remember that Marxism is theory, whereas Marxist-Leninism is a practical use of Marxism in the Russian social-economic landscape. A good example is the centralized leadership in Russia, because whereas Marxism believes that a revolution will create class consciousness (the understanding of class struggle) but Lenin didn't accept this, and the institutions and they had to deal with the varied ideologies and institutions of different organisations that existed during the Empire.
The main thing that people talk about being the difference between Marxism and Russian Communism, which developed during Lenin's rule of the party, is the Vanguard Party, the totalitarian centralized dictatorship of proletariat, rather than a more democratic and decentralized form of proletariat dictatorship (sounds funny doesn't it?) which Marx would have preferred.

The main difference between the ideologies rests in the perceived catalyst for revolution.

Karl Marx believed that the impetus for a revolution would come from the working classes themselves. He believed that poor working conditions, low pay and intolerable living conditions of the Proletariat would invoke a mass spontaneous revolt against the wealthy.

Lenin believed that revolution would only occur in the event of a leading group or 'Vanguard party' leading the poor into open revolt against their employers and masters.